For example, in an electrical inspection for a wafer on which a large number of integrated circuits are formed or a circuit device such as an electronic component, for example, a semiconductor device, there has been used a probe for an inspection which has inspection electrodes arranged in accordance with a pattern corresponding to a pattern of electrodes to be inspected in a circuit device to be inspected.
Conventionally, there has been used a probe for an inspection in which the inspection electrodes formed by a pin or a blade are arranged.
However, in the case in which the circuit to be inspected is a wafer having a large number of integrated circuits formed thereon, it is necessary to arrange a large number of inspection electrodes when fabricating a probe for an inspection to inspect the wafer. Therefore, the probe for an inspection is very expensive. Moreover, in the case in which the pitch of the electrodes to be inspected is small, it is hard to carry out the fabrication of the probe for an inspection itself.
Furthermore, a warpage is generally generated on the wafer and the state of the warpage is also varied for each product (wafer). For this reason, it is practically hard to cause each of the inspection electrodes of the probe for an inspection to come in contact with a large number of electrodes to be inspected in the wafer stably and reliably.
For the above reasons, in recent years, there has been proposed a probe for an inspection which serves to inspect an integrated circuit formed on a wafer, comprising a circuit board for an inspection on which a plurality of inspection electrodes is formed in accordance with a pattern corresponding to a pattern of electrodes to be inspected on a surface, an anisotropically conductive sheet disposed on a surface of the circuit board for an inspection and a sheet-like probe in which a plurality of electrode structures extended to penetrate in a direction of a thickness thereof is arranged in a flexible insulating sheet (for example, see Patent Document 1).
FIG. 39 is an explanatory sectional view showing a structure according to an example of a conventional probe card comprising a circuit board 85 for an inspection, an anisotropically conductive sheet 80 and a sheet-like probe 90.
In the probe card, there is provided a circuit board 85 for an inspection having a large number of inspection electrodes 86 formed in accordance with a pattern corresponding to a pattern of electrodes to be inspected in a circuit device to be inspected over a surface, and a sheet-like probe 90 is disposed on the surface of the circuit board 85 for an inspection through the anisotropically conductive sheet 80.
The anisotropically conductive sheet 80 has a pressurizing conducting portion exhibiting a conductivity in only a direction of a thickness or exhibiting the conductivity in only the direction of the thickness when a pressurization is carried out in the direction of the thickness. There have been known the anisotropically conductive sheets having various structures. For example, Patent Document 2 and the like have disclosed an anisotropically conductive sheet obtained by uniformly dispersing a metal particle in an elastomer (which will be hereinafter referred to as a “dispersion type anisotropically conductive sheet”).
Moreover, Patent Document 3 and the like have disclosed an anisotropically conductive sheet obtained by unevenly distributing a conductive magnetic particle into an elastomer, thereby forming a large number of conductive portions extended in a direction of a thickness and an insulating portion for mutually insulating them (which will be hereinafter referred to as an “an uneven distribution type anisotropically conductive sheet”) . Furthermore, Patent Document 4 and the like have disclosed the uneven distribution type anisotropically conductive sheet in which a step is formed between a surface of a conducting portion and an insulating portion.
The sheet-like probe 90 has a flexible insulating sheet 91 formed of a resin, for example, and has such a structure that a plurality of electrode structures 95 extended in a direction of a thickness is disposed on the insulating sheet 91 in accordance with a pattern corresponding to a pattern of electrodes to be inspected in a circuit device to be inspected.
Each of the electrode structures 95 has such a structure that a projected surface electrode portion 96 exposed from a surface of the insulating sheet 91 and a plate-shaped back electrode portion 97 exposed from a back of the insulating sheet 91 are integrally coupled through a short circuit portion 98 extended to penetrate through the insulating sheet 91 in a direction of a thickness thereof.
Such a sheet-like probe 90 is generally manufactured in the following manner.
First of all, a laminate material 90A obtained by forming a metal layer 92 over a surface of the insulating sheet 91 is prepared as shown in FIG. 40(a) and a through hole 98H penetrating in a direction of a thickness is formed on the insulating sheet 91 as shown in FIG. 40(b).
As shown in FIG. 40(c), subsequently, a resist film 93 is formed on the metal layer 92 of the insulating sheet 91, and furthermore, an electrolytic plating treatment is carried out by setting the metal layer 92 to be a common electrode. Consequently, the through hole 98H of the insulating sheet 91 is filled with a metal deposit so that the short circuit portion 98 coupled integrally with the metal layer 92 is formed, and furthermore, a projected surface electrode portion 96 coupled integrally with the short circuit portion 98 is formed on a surface of the insulating sheet 91.
Then, the resist film 93 is removed from the metal layer 92, and furthermore, as shown in FIG. 40(d), a resist film 94A is formed on the surface of the insulating sheet 91 including the surface electrode portion 96 and a resist film 94B is formed on the metal layer 92 in accordance with a pattern corresponding to a pattern of a back electrode portion to be formed, thereby carrying out an etching treatment over the metal layer 92. As shown in FIG. 40(e), consequently, an exposed portion in the metal layer 92 is removed so that the back electrode portion 97 is formed. Thus, the electrode structure 95 is formed.
Thereafter, the resist film 94A formed on the insulating sheet 91 and the surface electrode portion 96 is removed, and furthermore, the resist film 93 formed on the back electrode portion 97 is removed. Consequently, the sheet-like probe 90 is obtained.
In the probe for an inspection, the surface electrode portion 96 of the electrode structure 95 in the sheet-like probe 90 is provided so as to be positioned on an electrode to be inspected in the wafer over a surface of the wafer, for example, in the circuit device to be inspected.
In this state, the wafer is pressed by the probe for an inspection. Consequently, the anisotropically conductive sheet 80 is pressed by the back electrode portion 97 of the electrode structure 95 in the sheet-like probe 90.
Consequently, a conducting path is formed in a direction of a thickness between the back electrode portion 97 and the inspection electrode 86 of the circuit board 85 for an inspection over the anisotropically conductive sheet 80. As a result, an electrical connection between the electrode to be inspected in the wafer and the inspection electrode 86 of the circuit board 85 for an inspection can be achieved.
Then, a predetermined electrical inspection is executed for the wafer in this state.
According to such a probe for an inspection, the anisotropically conductive sheet 80 is deformed according to the degree of the warpage of the wafer when the wafer is pressed by the probe for an inspection. Therefore, it is possible to reliably achieve an excellent electrical connection for each of a large number of electrodes to be inspected in the wafer.
However, the probe for an inspection has the following problems.
In a process for forming the short circuit portion 98 and the surface electrode portion 96 in the method of manufacturing the sheet-like probe 90 described above, a plated layer formed by the electrolytic plating grows isotropically. As shown in FIG. 41, therefore, a distance W from a peripheral edge of the surface electrode portion 96 to that of the short circuit portion 98 is equivalent to a projection height h of the surface electrode portion 96 in the surface electrode portion 96 which is obtained.
Accordingly, a diameter R of the surface electrode portion 96 which is obtained exceeds a double of the projection height h and is considerably great.
For this reason, in the case in which the electrode to be inspected in the circuit device to be inspected is minute and is disposed at a very small pitch, a distance between the adjacent electrode structures 95 cannot be maintained sufficiently. As a result, in the sheet-like probe 90 which is obtained, the flexibility of the insulating sheet 91 is lost. For this reason, it is hard to achieve a stable electrical connection to the circuit device to be inspected.
In the electrolytic plating treatment, moreover, it is actually difficult to supply a current having an even current density distribution to the whole surface of the metal layer 92. Due to the uneven current density distribution, a growth speed of a plated layer is varied for each through hole 98H of the insulating sheet 91. Therefore, a great variation is generated in the projection height h of the surface electrode portion 96 which is formed and the distance W from the peripheral edge of the surface electrode portion 96 to that of the short circuit portion 98, that is, the diameter R.
In the case in which the projection height h of the surface electrode portion 96 has a great variation, it is hard to carry out a stable electrical connection to the circuit device to be inspected. On the other hand, in the case in which the diameter of the surface electrode portion 96 has a great variation, there is a possibility that the adjacent surface electrode portions 96 might be short-circuited.
In the foregoing, there is means for reducing the projection height h of the surface electrode portion 96. For the means to reduce the diameter of the surface electrode portion 96 which is thus obtained, there can be supposed means for reducing a diameter r of the short circuit portion 98 (which indicates the smallest length in the case in which a sectional shape is not circular), that is, a diameter of the through hole 98H of the insulating sheet 91. In a sheet-like probe obtained by the former means, however, it is hard to reliably achieve a stable electrical connection to the electrode to be inspected.
On the other hand, it is hard to carry out the formation itself of the short circuit portion 98 and the surface electrode portion 96 through the electrolytic plating treatment by the latter means.
In order to solve such problems, there have been proposed sheet-like probes obtained by disposing a large number of electrode structures having a tapered surface electrode portion which has a smaller diameter from a base end toward a tip respectively in Patent Document 5 and Patent Document 6.
The sheet-like probe described in the Patent Document 5 is manufactured in the following manner.
As shown in FIG. 42(a), there is prepared a laminate material 90B obtained by forming a resist film 93A and a surface side metal layer 92A on a surface of an insulating sheet 91 in this order and laminating a back side metal layer 92B on a back face of the insulating sheet 91.
As shown in FIG. 42(b), then, there is formed a through hole communicating with each of the back side metal layer 92B, the insulating sheet 91 and the resist film 93A in the laminate material 90B and extended in a direction of a thickness.
Consequently, a concave portion 90K for forming an electrode structure which has a tapered configuration adapted to a short circuit portion and a surface electrode portion in an electrode structure to be formed is provided on a back face of the laminate material 90B.
As shown in FIG. 42(c), subsequently, a plating treatment is carried out by setting the surface side metal layer 92A in the laminate material 90B to be an electrode so that the concave portion 90K for forming an electrode structure is thus filled with a metal to form a surface electrode portion 96 and a short circuit portion 98.
Then, the back side metal layer 92B in the laminate material is subjected to an etching treatment and is thus removed partially. Consequently, a back electrode portion 97 is formed as shown in FIG. 42(d). Thus, the sheet-like probe 90 is obtained.
Moreover, the sheet-like probe described in the Patent Document 6 is manufactured in the following manner.
As shown in FIG. 43(a), there is prepared a laminate material 90C obtained by forming a surface side metal layer 92A on a surface of an insulating sheet material 91A having a greater thickness than an insulating sheet in a sheet-like probe to be formed and laminating a back side metal layer 92B on a back face of the insulating sheet material 91A.
As shown in FIG. 43(b), then, there is formed a through hole communicating with each of the back side metal layer 92B and the insulating sheet material 91A in the laminate material 90C and extended in a direction of a thickness. Consequently, a concave portion 90K for forming an electrode structure which has a tapered configuration adapted to a short circuit portion and a surface electrode portion in an electrode structure to be formed is provided on a back face of the laminate material 90C.
By carrying out a plating treatment with the surface side metal layer 92A in the laminate material 90C set to be an electrode, subsequently, the concave portion 90K for forming an electrode structure is thus filled with a metal to form a surface electrode portion 96 and a short circuit portion 98 as shown in FIG. 43(c).
Subsequently, the surface side metal layer 92A in the laminate material 90C is removed, and furthermore, the insulating sheet material 91A is subjected to an etching treatment, thereby removing a surface side portion of the insulating sheet. As shown in FIG. 43(d), thus, the insulating sheet material 91 having a predetermined thickness is formed and the surface electrode portion 96 is exposed.
Then, the back side metal layer 92B is subjected to the etching treatment so that the back electrode portion 97 is formed. Thus, the sheet-like probe 90 is obtained as shown in FIG. 43(e).
According to such a sheet-like probe 90, the surface electrode portion 96 is tapered. Therefore, the surface electrode portion 96 having a small diameter and a great projection height can be formed in a state in which a distance from the surface electrode portion 96 of an adjacent electrode structure is maintained sufficiently. Furthermore, each of the surface electrode portions 96 of the electrode structure 95 is formed by setting, as a cavity, the concave portion 90K for forming an electrode structure which is provided on the laminate material. Consequently, it is possible to obtain the electrode structure 95 having a small variation in the projection height of the surface electrode portion 96.
In these sheet-like probes, however, the diameter of the surface electrode portion in the electrode structure is equal to or smaller than the diameter of the short circuit portion, that is, the diameter of the through hole formed on the insulating sheet. Therefore, the electrode structure slips from the back face of the insulating sheet. Consequently, it is hard to actually use the sheet-like probe.
In order to solve the problem, for example, there has been proposed a sheet-like probe which has a holding portion on the surface electrode portion side in the electrode structure which has a small diameter and a tapered configuration described in the Patent Document 7 and prevents the electrode structure from slipping from the back face of the insulating sheet.
The sheet-like probe described in the Patent Document 7 is manufactured in the following manner.
As shown in FIG. 44(a), there is prepared a laminate material 132 including five layers formed by a surface side metal layer 122, an insulating sheet 124, a first back side metal layer 126, an insulating layer 128 and a second back side metal layer 130.
As shown in FIG. 44(b), an opening portion 134 is provided on the second back side metal layer 130 in the laminate material 132 and the insulating layer 128 is subjected to etching through the opening portion 134 so that a through hole 136 is provided on the insulating layer 128.
Next, the etching is carried out over the first back side metal layer 126 exposed from the bottom portion of the through hole of the insulating layer 128, thereby exposing the insulating sheet 124 from the bottom portion of the through hole 136.
Then, the insulating sheet 124 is subjected to the etching via the through hole 136 of the first back side metal layer 126, thereby exposing the surface side metal layer 122 from the bottom portion of the through hole 136.
Thus, the metal layer and the resin layer (the insulating layer 128 and the insulating sheet 124) are mutually subjected to the etching, thereby forming a through hole 138 communicating with each of the second back side metal layer 130, the insulating layer 128, the first back side metal layer 126 and the insulating sheet 124 and extended in a direction of a thickness. Thus, a concave portion 90K for forming an electrode structure having a tapered configuration adapted to a short circuit portion and a surface electrode portion of an electrode structure to be formed is provided on the back face of the laminate material 132.
Subsequently, a plating treatment is carried out by setting the surface side metal layer 122 in the laminate material 132 as an electrode. As shown in FIG. 44(c), consequently, the concave portion 90K for forming an electrode structure is filled with a metal to form the surface electrode portion 96 and the short circuit portion 98.
Then, the surface side metal layer 122 in the laminate material 132 is removed, and the insulating sheet 124 is subjected to the etching treatment and is thus removed to expose the first back side metal layer 126 (FIG. 44(d)).
Thereafter, the first back side metal layer 126 is subjected to the etching treatment to form a holding portion, and the first back side metal layer 126 is subjected to the etching treatment and is thus removed partially, thereby forming the back electrode portion 97 and a support portion. As shown in FIG. 44(e), thus, the sheet-like probe 90 is obtained.
In the sheet-like probe obtained by such a manufacturing method, furthermore, the back face of the laminate material 90C is provided with the concave portion 90K for forming an electrode structure which has a tapered configuration adapted to the short circuit portion and the surface electrode portion in the electrode structure to be formed. Therefore, a tip diameter 92T of the concave portion for forming an electrode structure is smaller than a diameter of an opening portion 92H formed on the back face of the laminate material 90C.
In this fabrication process of the sheet-like probe described in the Patent Document 7, a through hole for forming the electrode structure is formed in a insulating layer made of polyimide or the like. As shown in FIG. 45, the through hole can be obtained by forming the pattern of a photoresist film 140 including an opening 140a in the section in which the through hole will be formed on one face of the second back side metal layer 130 and by immersing the whole sheet in an etchant and carrying out an etching, thereby forming the through hole in the insulating layer 128 and the insulating sheet 124 made of polyimide.
In this method, a through hole 142a is formed in such a manner that a surface side metal layer 122 that has been laminated on the insulating sheet 124 is exposed at the bottom of the through hole 81a, and the electrode structure is formed through the process in which an electrolytic plating is carried out using the surface side metal layer 122 as a common electrode.
However, in the case in which a through hole is formed by carrying out an etching to the insulating layer 128 and the insulating sheet 124 made of polyimide, as shown in FIG. 45, the through hole 142a is tapered, and the diameter of the through hole becomes smaller as the through hole is deeper. Accordingly, in the case in which a polyimide film with a large film thickness is used, a through hole is closed before the through hole reaches to the surface side metal layer 122, thereby preventing the through hole 142a from being formed.
More specifically, in the case in which a through hole for forming the electrode structure is tried to be formed in a insulating layer made of polyimide by etching processing as a conventional method, there is a problem of that the through hole 142a that reaches to the surface side metal layer 122 cannot be formed in the case in which an film thickness t1 of the insulating sheet 124 and an film thickness t2 of the insulating layer 128 of which the surface is covered by a photoresist film 140 is thicker.
An etching treatment angle θ in an etching treatment, which differs in processing conditions, is said to be in the range of 45° to 55° in general.
Consequently, the through hole 142a cannot be reliably formed in the insulating layer if the sum of the film thickness t1 and the film thickness t2 of the insulating sheet 124 and the insulating layer 128 is more than half of a diameter φ1 of an opening.
Therefore, to reliably form the through hole 142a, it is necessary to decrease an film thickness t1 of the insulating sheet 124 or an film thickness t2 of the insulating layer 128. Accordingly, there is a case in which it is difficult to form the surface electrode portion having a large projection height. In the case in which a thickness of the electrode structure of the sheet-like probe is decreased and, for instance, the periphery of an electrode to be inspected in a circuit device to be inspected is covered by an insulating layer having a large thickness, it may be difficult to connect the electrode structure and the electrode to be inspected.
In this manufacturing method, a thickness of the electrode structure is almost equivalent to the sum of a thickness of the insulating sheet 124 and a thickness of the insulating layer 128, and a thickness of the short circuit portion is equivalent to a thickness of the insulating layer 128. Consequently, to increase a thickness of the electrode structure, it is necessary to increase a thickness of the insulating layer.
The sheet-like probe with an insulating layer having a large thickness has an effect of a high durability in repeated use. However, for such a sheet-like probe, a deformation amount of the insulating layer in a direction of a thickness is decreased, thereby decreasing a moving amount of the electrode structure in a direction of a thickness.
In particular, to suppress a positional shift of the electrode structure in a planar direction thereof, the sheet-like probe having a shape supporting the insulating film by a metal support material tends to further decrease a moving amount of the electrode structure in a direction of a thickness.
A decrease in a moving amount of the electrode structure in a direction of a thickness for the sheet-like probe causes a concave and convex absorbing ability of a probe card using the sheet-like probe to be reduced.
More specifically, for a probe card, the sum of a concave and convex absorbing ability of an anisotropically conductive connector and a concave and convex absorbing ability of the sheet-like probe becomes a concave and convex absorbing ability of the probe card. Consequently, a decrease in a concave and convex absorbing ability of the sheet-like probe causes a concave and convex absorbing ability of the probe card to be reduced.
In the case in which an electrode to be inspected in a circuit device has a variation in height for the probe card having a reduced concave and convex absorbing ability, a large pressure is required to achieve an electrical connection of the probe card to each electrode to be inspected. Accordingly, since there is increased a compressive deformation amount of a conductive portion in an anisotropically conductive connector, a permanent deformation of a conductive portion in an anisotropically conductive connector occurs early and an electrical connection is made difficult, thereby requiring an exchange of the anisotropically conductive connector and increasing an inspection cost.
In addition, in the case in which an electrode to be inspected in a circuit device is a solder bump electrode having a large variation in height, it may be impossible to achieve an electrical connection of the probe card to each electrode to be inspected.
Therefore, it is desired to increase a moving amount of the electrode structure in a direction of a thickness for the sheet-like probe to obtain a probe card having a large concave and convex absorbing ability.
As a means of increasing a concave and convex absorbing ability, a sheet-like connector as shown in the Patent Document 8 is known for instance.
Such a sheet-like connector is an anisotropically conductive connector composed of a complex conductive sheet in which a tapered movable conductor adapted to a tapered through hole formed in an insulating sheet is formed movably in a direction of a thickness of an insulating sheet and two anisotropically conductive elastomer sheets disposed on one and another faces of the complex conductive sheet.
For such an anisotropically conductive connector having a complex conductive sheet, a movable electrode in the complex conductive sheet can be moved in a direction of a thickness. Therefore, in the case in which the movable electrode is pressurized in a direction of a thickness, the two anisotropically conductive elastomer sheets disposed on one and another faces of the complex conductive sheet are compressed and deformed linking to each other. As a result, the total concave and convex absorbing ability of the two anisotropically conductive elastomer sheets appears as a concave and convex absorbing ability of the anisotropically conductive connector, thereby obtaining a high concave and convex absorbing ability.
Moreover, a thickness required for obtaining the desired concave and convex absorbing ability can be ensured by the total thickness of the two anisotropically conductive elastomer sheets, and an individual anisotropically conductive elastomer sheet having a small thickness can be used, thereby obtaining a high resolution.
However, the above described anisotropically conductive connector has a following problem in practice.
For the above described anisotropically conductive connector, a movable conductor in the complex conductive sheet is supported by both the insulating sheet and the anisotropically conductive elastomer sheet. In the case in which the complex conductive sheet and the anisotropically conductive elastomer sheet are separated from each other, the movable conductor may slip from the insulating sheet. Consequently, it is extremely hard to actually use the complex conductive sheet individually.
Accordingly, in the case in which a failure occurs in either of the complex conductive sheet and the anisotropically conductive elastomer sheet in the anisotropically conductive connector, either of the complex conductive sheet and the anisotropically conductive elastomer sheet cannot be exchanged to new one but the whole anisotropically conductive connector must be exchanged to new one.
Moreover, to form the movable conductor and its projecting portion in the above described sheet-like connector, a laser processing is carried out from the side of the insulating sheet to a laminate material in which an insulating sheet material and an auxiliary layer for forming a projecting portion are laminated, and a tapered through hole is formed.
Therefore, as described in a formation of the concave portion 90K for forming an electrode structure shown in the Patent Document 7, the sheet-like connector has problems of a miniaturization in a diameter of a tip portion of the movable conductor in the case in which a processing is carried out to a laminate sheet having a large thickness and of ensuring an insulating property between adjacent movable conductors in the case in which an electrode pitch of a wafer to be inspected described later is decreased. As a result, there are problems in the case in which a sheet-like connector is used as a sheet-like probe for inspecting a wafer.
As an electrode pitch of a wafer to be an object to be inspected becomes shorter, a disposition pitch of an electrode structure of the sheet-like probe becomes shorter. Although a disposition pitch of the electrode structure is generally in the range of 100 to 120 μm at present, it is thought that the disposition pitch must be for instance less than 100 μm in the future and 80 μm or less in the further future.
On the other hand, to ensure an insulating property between adjacent electrode structures, a width of an insulating portion between the electrode structures must be for instance in the range of 40 to 50 μm (a difference between a disposition pitch of an electrode structure and a diameter φ1 of an opening).
In the case in which a thick polyimide film is used to ensure the strength of the polyimide film, a diameter φ1 of an opening must be larger to form a through hole by etching as described above. However, in the case in which a diameter φ1 of an opening is made larger while a disposition pitch of an electrode structure is kept constant, an insulating property between adjacent electrode structures cannot be ensured.
Accordingly, in the case in which a disposition pitch of an electrode structure is made smaller, a thickness of a polyimide film is restricted. For instance, in the case in which a disposition pitch of an electrode structure is 120 μm and a diameter φ1 of an opening of a through hole is 70 μm, a thickness t of a polyimide film to be used must be 35 μm or less. In addition, in the case in which a diameter φ2 of an opening at a bottom side is made a certain level or more, the thickness t must be further less.
If the insulating layer 128 with a thickness of 50 μm is tried to be used to make the strength of the insulating layer 128 higher, a diameter φ1 of an opening of a through hole must be 100 μm or larger, thereby preventing an insulating property between adjacent insulating layers of electrode structures to be manufactured from being ensured. Consequently, it is impossible to make a diameter of an opening larger corresponding to a thickness of the insulating layer 128.
In the case in which an electrode structure is formed in a tapered through hole 142a as shown in FIG. 45, since a diameter φ2 of an opening at an end side in an etching direction is smaller, an electric resistance value increases. Accordingly, it is preferable that a diameter φ2 of an opening at the small diameter section is as large as possible.
Moreover, in the case in which the diameter φ2 of an opening is small, the small diameter section affects the electric resistance value. As a result, there is a problem that a variation of electric resistance values between electrode structures formed in the sheet-like probe may become larger.
Patent Document 1: Japanese Laid-Open Patent Publication No. 1995-231019
Patent Document 2: Japanese Laid-Open Patent Publication No. 1976-93393
Patent Document 3: Japanese Laid-Open Patent Publication No. 1978-147772
Patent Document 4: Japanese Laid-Open Patent Publication No. 1986-250906
Patent Document 5: Japanese Laid-Open Patent Publication No. 1999-326378
Patent Document 6: Japanese Laid-Open Patent Publication No. 2002-196018
Patent Document 7: Japanese Laid-Open Patent Publication No. 2004-172589
Patent Document 8: Japanese Laid-Open Patent Publication No. 2001-351702